Which companies offer genealogy DNA testing services?

There are several DNA tests that can be used to explore your genealogy. For example, there are Y-DNA tests which test the Y chromosome found only in males. There are also mtDNA tests which examine the mitochondrial DNA, a small circle of DNA that we inherit only from our mothers. And there are autosomal DNA tests, which examine the DNA of your 23 chromosomes. The cost of these tests has come down considerably in the last few years.

There is much more information about the different types of tests, and the different testing companies, at the International Society of Genetic Genealogy ("ISOGG") wiki located at http://www.isogg.org/wiki.